In recent years, attention has been given to electric-powered vehicles such as electric vehicles, hybrid vehicles and fuel cell vehicles against the background of environmental issues. These electric-powered vehicles are equipped with an AC motor driven by an inverter as a motive power source for vehicle running.
In these electric-powered vehicles, when a drive torque balances a vehicle weight, e.g., during slope climbing and thus the vehicle enters a so-called “motor lock state” in which the AC motor attains an extremely low motor rotation number while outputting a motor torque, a current concentratedly flows in a specific phase so that a loss increases in the inverter, and the inverter may overheats within a short time.
Accordingly, Japanese Patent Laying-Open No. 2002-272125 (patent document 1) has disclosed a control device that can reduce a loss in a switching element forming an electric converter when a frequency of an AC voltage reference is equal to a predetermined value or less. This control device performs PWM (Pulse Width Modulation) control on a power semiconductor element that forms an electric power converter. When the frequency of the AC voltage reference is equal to the predetermined value or less, the control device adds a DC (direct current) offset voltage to the AC voltage reference, and thereby reduces a loss in the power semiconductor element.
This control device can reduce the loss in the power semiconductor element when the AC low-frequency power is being supplied (see Patent Document 1).
Patent Document 1: Japanese Patent Laying-Open No. 2002-272125
Patent Document 2: Japanese Patent Laying-Open No. 9-191508
Patent Document 3: Japanese Patent Laying-Open No. 2000-102257
Patent Document 4: Japanese Patent Laying-Open No. 6-245577
Patent Document 5: Japanese Patent Laying-Open No. 2004-48885
Patent Document 6: Japanese Patent Laying-Open No. 2003-204028